Induction motors are widely used electric motors. Such motors include a fixed stator that is mounted inside the motor casing. The stator comprises a series of stationary windings with a large cylindrical opening in the center. A rotor having a series of windings mounted on a rotatable shaft is mounted inside the stator opening. The rotor is free to revolve within the opening. Application of electric power to the stator produces a rotating magnetic field in the stator, which induces high currents in the rotor. The rotor currents, in turn, produce their own magnetic fields, which interact with the main field generated in the stator and make the rotor turn.
Induction motors are easy to manufacture and are essentially trouble-free in actual service. In a practical induction motor, the rotor consists of a laminated iron core which is slotted lengthwise all around its periphery. Solid bars of aluminum, copper or other conductors are tightly pressed or imbedded into the slots. At both ends of the rotor, short-circuiting rings are welded or brazed to the bars to make a solid structure for placement on the iron core. The short circuiting rings actually form shorted turns that have currents induced in them by the field flux. When assembled, the periphery of the rotor is separated from the stator by a very small air gap. The width of the air gap is as small as mechanically possible to ensure that the strongest possible electromagnetic induction takes place.
For the induction motor to run, the rotor must rotate at a slightly slower speed than the rotating magnetic field of the stator. If the rotor turned at exactly the same speed as the stator field, there would be no relative difference in motion between the field and the rotor. The bars in the rotor would not cut the field flux lines and no current would be induced in the rotor. The difference between the field and the rotor speed is known as slip. Rotors tend to slip behind the field speed by two to ten percent and the slip will increase as the load on the motor increases.
Three-phase inductive motors, as well as many other three-phase loads, are susceptible to damage due to momentary power supply faults, such as are caused by line power loss and buss transfers. Simply put, this means that to momentarily shut off the power to an operating electric motor and then turn the power back on again while the motor is still spinning may cause damage to the motor and to the equipment that the motor was driving at the time of the power interrupt. A procedure is needed, therefor, to detect when the power has been turned off and to disconnect the motor from the power line before the power is restored, so that when the power is restored., it is not fed to the motor. By doing this, damage to the motor and the equipment that is being powered by the motor is avoided.
As indicated, the purpose of momentary power loss protection is to protect the inductive motor and related components from the damaging effects of power interruptions. Related components that can be damaged by a power interruption may include compressor impellers, shaft key ways, and starter contacts.
Power interruptions can result from loss of external line power and internal buss transfers. An external line power failure is failure of the power as it comes from the power utility generating station. Such failures may occur due to weather effects on the line transmission equipment or from the switching of generator sources at the power utility, or from a failure at the power utility. An internal buss transfer comprises a switch from one power source to another, such as might occur when automatically switching from external utility line power to a backup generator at the facility in which the motor is installed. It should be remembered that the types of faults that this invention is concerned with are those in which there is a momentary loss of power followed virtually immediately by a restoration of power.
Momentary power interruptions may be unexpected as in the case of power line switching closures, brownouts and substation transfers. They may also be expected, resulting from buss transfers to an alternate power source within a facility. Whether expected or unexpected, interruptions result in large transient currents up to twelve times full load amperage and transient torques up to twelve times full load torque or twenty times full load torque if power factor correction capacitors are used in conjunction with the induction motor.
In inductive motors, the reconnection currents that are presented due to a momentary power loss generate large electromagnetic forces between neighboring stator windings as well as between the rotor bars. This exposes the stator end windings and the rotor shorting rings to excessive stress. This stress has been identified through studies and experience as causing or accelerating motor failures. Failure of the motor is typically not immediate but is most often seen as accelerated wear and greatly reduced motor life.
The large transient torques that result from the loss of power and quick reapplication of the power may be negative at times. Such negative torques may attempt to reverse the direction of rotation of the motor and cause damage to the equipment that is being powered by the motor. Impellers that may be powered by an induction motor, for instance, are often affixed to the shaft on which the impellers rotate by aligned grooves or key ways in the shaft and the impeller. A key is inserted in the key ways to rotationally mate the impeller to the shaft. Slapping of the key ways and exposure of the impellers to large mechanical stresses can occur as the result of a momentary power loss that causes the motor to try to reverse direction of rotation by the transient torques.
The above potentially damaging interrupts need to be distinguished from noise on the line and from the loss of a single phase of the three-phase power. Noise can be the result of weather effects on the power received from the power lines, such as may be caused by an electrical storm or the switching on and off of nearby machines, and is usually a transient condition. Shutdowns of the motor as a result of detecting such occurrences as noise are considered nuisance shutdowns. Three phase power is alternating power and requires three lines to deliver the power. On occasion, one phase of the power is lost. In such instances, the motor will continue to operate, but in a single phase mode of operation. A single phase loss may cause motor damage and is detected using a separate and slower mechanism.
Although a wide variety of fault detectors for use in three-phase circuits are presently available, the time response window of conventional fault detectors is inadequate. Some conventional detectors respond too slowly to critical faults and damage can quickly occur when the detector does not interrupt the power supply before power is restored. Other detectors respond too quickly to less critical faults where a slower response would avoid false alarms.
Moreover, in many applications, conventional fault detectors do not distinguish a momentary power loss from noise. As a result, such detectors needlessly trip the motor off in the presence of mere noise on the lines. A false trip may also occur in the event of a loss of one phase of the line power. It is desirable to absolutely minimize such false trips, since they are costly in the maintenance actions required to reinitialize the motor and the equipment powered by the motor and in lost time for the work which the motors are intended to be performing.
The ultimate goal of momentary power loss protection circuits is to disconnect the motor from the power line before the duration of the interruption allows the reconnection currents and torques to achieve a damaging level. For very short power interruptions (defined here as less than 1 to 2 line cycles), the reconnection transients are less than those seen for a normal start of the motor and, therefore, are not destructive to the motor. It is just as well that the motor not be decoupled in the event of such short duration power losses so that the motor will be powered again when power is restored after the short interrupt. Since the motor is not injured by the reapplication of power, this avoids an unnecessary shutdown.
Likewise, for very long power interruptions (defined here as greater than six seconds), the reconnection transients are also below the transients seen for a normal start of the motor. As is indicated above, in cases where the reconnection transient is less than the current and torque experienced during a normal start-up, the motor will not be injured by the reconnection and it is desirable to permit such reconnection to occur in order to avoid an unnecessary shutdown.
Power interruptions of a duration between the above defined short and long power interruptions result in reconnections which are potentially destructive to the motor. Optimal momentary power loss protection would protect the motor from reconnections falling within that window of time, but would not permanently disconnect power when momentary power losses of a duration outside of the designated time window are detected. No adequate momentary power loss protection for the defined duration currently exists. The main function of the present invention is to disconnect the motor from the line in the event of power interruptions lasting for more than one or two line cycles. Thus when a long interruptions occurs, the motor has already been removed from the line power source.
It is accordingly an object of the present invention to provide a momentary power loss detection system that is capable of distinguishing faults that occur in a time regime between short duration power faults and long duration power faults.
It is another object of the present invention to provide a momentary power loss detector that is microcomputer based, thereby minimizing the number of discrete electrical elements necessary to perform the detection function.
Still another object of the present invention is to provide a momentary power loss detection system in which robustness of the data is inherent.
These and other objects of the invention will be apparent from the attached drawings and the description of the preferred embodiment which follow hereinbelow.